In line with an increase in the price of energy sources due to the depletion of fossil fuels and amplification of interests in environmental pollution, environmentally-friendly alternative energy sources have become an indispensable factor for the future life.
In particular, the demand for secondary batteries as an environmentally-friendly alternative energy source has rapidly increased as the technology development and demand for mobile devices have increased.
Typically, lithium metal has been used as a negative electrode of the secondary battery, but, since a battery short circuit may occur due to the formation of dendrites and there is a risk of explosion due to the short circuit, the use of a carbon-base active material capable of reversibly intercalating and deintercalating lithium ions as well as maintaining structural and electrical properties has emerged.
Various types of carbon-based materials including artificial graphite, natural graphite, and hard carbon has been applied to the carbon-based active material, and, among these materials, a graphite-based active material, which may ensure life characteristics of a lithium secondary battery due to excellent reversibility, has been the most widely used. Since the graphite-based active material has a low discharge voltage versus lithium of −0.2 V, a battery using the graphite-based active material may exhibit a high discharge voltage of 3.6 V, and thus, the graphite-based active material provides many benefits in terms of energy density of the lithium battery.
Recently, in order to prepare a lithium secondary battery having excellent output characteristics at room temperature and low temperature, a method of reducing charge transfer resistance of lithium ions in a lithium secondary battery has emerged. For this purpose, a method of preparing an electrode active material with a nanometer size has been proposed. With respect to this method, {circle around (1)} high output characteristics may be obtained because a relative movement distance of lithium ions is reduced by the nano-sized active material, or {circle around (2)} a rapid electrochemical reaction may be expected because a contact with an electrolyte is facilitated due to a high surface area of the nano-sized active material. Furthermore, {circle around (3)} an effect of improving a diffusion rate of lithium ions may be obtained because pores present between nano-sized active material particles provide a space for the expansion of the electrode active material.
However, in a case in which a nano-sized negative electrode active material is prepared, charge transfer resistance is increased due to polycrystallinity of the nanostructure, or intercalation and deintercalation reactions of lithium ions become difficult due to components (organic film, inorganic film) of cylindrical solid electrolyte interphase (SEI) formed on the surface of an electrode. Accordingly, since the charge transfer resistance is increased, it is disadvantageous in that degradation of lithium ion battery performance occurs.
Thus, there is a need to develop a graphite-based negative electrode active material having low resistance (high output) at room temperature and low temperature.